Do the same amount of time walking and the same amount of time sitting - so if you want to do 5 hours (extremly hard but if you think you can - ) it will be 2.5 hours walking 2.5 hours sitting which you can cut to different meditation segments however you think is right you can do 20 walking 20 sitting - or 30 +30 or start with 10+10 then move to 20+20 and if gets hard do 10+10

Walk first, then sit. If you cannot sit for a full hour, you can probably walk for a full hour without difficulty. So walk, then sit as long as you can. When it gets too painful, change your posture once, but not more than once, then get up and walk again. If you feel sleepy, get up and walk. If the legs feel stiff after walking for a full hour, you're doing it right.